This invention relates to a process for fixing an image, and to an imaging medium for use in this process. More specifically, this invention relates to a process capable of fixing images produced by the aforementioned '489, '612 and indicator sensitizing dye processes, although the fixing process of the invention may also be useful for fixing images produced by other processes but having unchanged superacid precursor present in the non-exposed areas of the final image.
As discussed in the aforementioned applications, some conventional non-silver halide photosensitive compositions, for example photoresists, contain molecules which are inherently photosensitive, so that absorption of a single photon brings about decomposition of only the single molecule which absorbs the photon. However, a dramatic increase in the sensitivity of such photosensitive compositions can be achieved if the photosensitive molecule initiates a secondary reaction which is not radiation-dependent and which effects conversion of a plurality of molecules for each photon absorbed. For example, photoresist systems are known in which the primary photochemical reaction produces an acid, and this acid is employed to eliminate acid-labile groups in a secondary, radiation-independent reaction.
U.S. Pat. No. 4,916,046 describes a positive radiation-sensitive mixture using a monomeric silylenol ether, and a recording medium produced therefrom. This patent also contains an extensive discussion of radiation-sensitive compositions which form or eliminate an acid on irradiation. According to this patent, such radiation-sensitive compositions include diazonium, phosphonium, sulfonium and iodonium salts, generally employed in the form of their organic solvent-soluble salts, usually as deposition products with complex acids such as tetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroantimonic acid and hexafluoroarsenic acid; halogen compounds, in particular triazine derivatives; oxazoles, oxadiazoles, thiazoles or 2-pyrones which contain trichloromethyl or tribromomethyl groups; aromatic compounds which contain ring-bound halogen, preferably bromine; a combination of a thiazole with 2-benzoylmethylenenaphthol; a mixture of a trihalomethyl compound with N-phenylacridone; .alpha.-halocarboxamides; and tribromomethyl phenyl sulfones.
The aforementioned phosphonium, sulfonium and iodonium salts are superacid precursors which, upon exposure to ultraviolet radiation, decompose to produce superacids, that is to say acids with a pK.sub.a less than about 0. Other materials decompose to produce superacids in a similar manner.
The applications and patents mentioned in the "References to Related Applications and Patents" section hereof describe processes in which imagewise breakdown of a superacid precursor to produce unbuffered superacid is effected using imagewise exposure of the medium to radiation of a wavelength which, in the absence of the sensitizer, will not cause breakdown of the superacid precursor. The imagewise distribution of unbuffered superacid thus produced is used to catalyze the thermal breakdown of the secondary acid generator, thus producing, in the exposed areas of the medium, a molar quantity of the secondary acid larger than the molar quantity of unbuffered superacid present in these areas. The secondary acid is used to bring about the absorption change (hereinafter sometimes called "color change," with the understanding that the "color" may not be in the wavelength range visible to the human eye) of the indicator dye, thus forming an image.
At least the non-exposed areas of the image thus produced still contain unchanged superacid precursor, and this superacid precursor is still susceptible to decomposition by electromagnetic radiation with formation of acid. Such post-imaging generation of acid will also occur in the exposed areas if, as is usually the case, unchanged superacid precursor is also present in these areas. Unfortunately, in practice it is usually necessary to incorporate into this type of imaging medium a molar quantity of superacid precursor substantially larger than the molar quantity of superacid required for imaging, since the quantum yield for the generation of superacid during the imagewise exposure (and hence the overall sensitivity of the imaging process) is dependent upon the concentration of superacid precursor present, and a substantial excess of superacid precursor is needed for optimum quantum yield. In fact, the optimum molar quantity of superacid precursor may be a substantial fraction of the molar quantity of secondary acid generated by complete thermal breakdown of the secondary acid generator. For example, the aforementioned U.S. Pat. No. 5,286,612 describes a '612 imaging medium containing 1 mole of sensitizing dye, 5 moles of superacid precursor and a quantity of secondary acid generator sufficient to yield 10 moles of secondary acid. Accordingly, most images produced using media of the '612 or indicator sensitizing dye types are susceptible to post-imaging changes due to unwanted superacid precursor decomposition and acid generation in non-exposed areas.
The present invention relates to a process in which the superacid precursor remaining in at least the unexposed areas of the images produced by the methods discussed above is destroyed, thus fixing the image.